1. Field of the Invention
The present invention relates to a visible and infrared light photographing lens system, and in particular, to a visible and infrared light photographing lens system which enables the same object to be photographed simultaneously using a visible light and an infrared light.
2. Description of the Related Art
If a television camera (for example, a CCTV camera) is used to photograph images for monitoring or other applications, the photographing is carried out with a visible light in a bright environment such as in the daytime and with an infrared (near infrared) light in a dark environment such as in the nighttime when an object cannot be observed.
In the prior art, a single camera may be used to carry out both visible light photographing and infrared light photographing. A camera is known in which a photographing lens installed in the camera main body need not be changed from one for a visible light to one for an infrared light. For example, if a photographing lens designed for photographing with a visible light is used directly for photographing with an infrared light, the infrared light may disadvantageously increase the magnitude of axis chromatic aberrations. In particular, if a photographing lens for a visible light having a zoom function is used for an infrared light, when a zoom magnification is changed, the object may be out of focus even if tracking adjustment (flange back adjustment) is carried out. Thus, disadvantageously, the tracking adjustment (flange back adjustment) cannot be effectively carried out.
Accordingly, if a photographing lens for a visible light is also used for photographing with an infrared light, the following method is applied: a method of optically reducing the magnitude of color aberrations down to an infrared wavelength region using low-dispersion glass such as fluorite or ED glass for a lens or a method of inserting an optical member (lens, prism) into an optical system of the photographing lens to compensate for the color aberrations.
Further, under a little dark photographing conditions such as in the evening, it may be desirable to simultaneously execute photographing with an infrared light which enables the object to be clearly photographed and photographing with a visible light which provides color information on the object. However, if a single camera is switched between visible light photographing and infrared light photographing, these photographing operations cannot be simultaneously preformed. Thus, in the prior art, two cameras for visible and infrared lights, respectively, may be used to execute simultaneous photographing. However, if the two cameras are used, they must be simultaneously operated in carrying out focusing, zooming, or the like so that their photographing distances (the distance to the object position at which the camera can be focused on the object (object distance)) or angles of view are equal to each other. This requires much time and effort. Further, the use of the two cameras may result in parallax between the cameras. In this case, the angle of view of a video photographed using a visible light may not be equal to the angle of view of a video photographed using an infrared light.
In contrast, Japanese patent application Publication No. 2003-69865 proposes an image pickup apparatus in which a half mirror divides an object light incident on a photographing lens, into an object light for a visible light and an object light for an infrared light which are used to pick up an image. This apparatus can make the angle of view of a video photographed using a visible light equal to the angle of view of a video photographed using an infrared light.
Alternatively, it is possible to use two cameras for visible and infrared lights, respectively, and divide an object light from an object to be photographed into an object light for a visible light and an object light for an infrared light using a mirror or a prism. In this case, the object lights obtained are allowed to enter photographing lens of the visible and infrared light cameras, respectively, so as to make the angle of view of a video photographed with the visible light camera equal to the angle of view of a video photographed with the infrared light camera.